currentTransaction.startSpan return null when I use express.

Describe the bug

const currentTransaction = apm.currentTransaction;
const span = currentTransaction.startSpan(name);

I use the code above in express context, but the span is null.

Environment (please complete the following information)

• OS: [e.g. Linux] win10
• Node.js version: 12.18.1
• APM Server version:3.6.1
• Agent version: 6.3.2

How are you starting the agent? (please tick one of the boxes)

• [√] Calling agent.start() directly (e.g. require('elastic-apm-node').start(...))
• Requiring elastic-apm-node/start from within the source code
• Starting node with -r elastic-apm-node/start

[ancms2600]

Thank you @astorm. These details are good to know. Perhaps they can be added to the API docs? https://www.elastic.co/guide/en/apm/agent/nodejs/master/transaction-api.html#transaction-start-span

[astorm]

@ancms2600 At the time of this writing there’s a handful of reasons that the agent might not be able to start a span (and, as a result, return null).

First – if there’s no current transaction when you call apm.startSpan then the agent will generate no spans. This check occurs here.

Second – if you’re sampling spans (say, via the ELASTIC_APM_TRANSACTION_SAMPLE_RATE configuration) and a transaction ends up being sampled then the agent will generate no spans. You can see this check here.

Third – if the current transaction has ended no further spans will be generated until a new transaction starts. You can see this check here.

Fourth – If you’re using the ELASTIC_APM_TRANSACTION_MAX_SPANS configuration and you’ve hit the maximum number of spans for a transaction, the agent will generate no further spans. You can see this check here.

It’s hard to say why this is happening in your specific case, but if it’s happening randomly, then my guess would be your sampling rate. You should always presume that startSpan may not return a span object. (in other words, your guard clause is appropriate)